The Mizuki Diving Club, or MDC, is in trouble: its longtime sponsor has passed away, and the people who now run his corporation do not care for diving nearly as much as he did. The only way they will continue to finance the MDC is if it achieves a seemingly impossible goal, namely earning a place at the 2018 Tokyo Olympics.

There is a device, called an SCM, which can effectively turn one person into another's slave. It fits into a person's mouth, much like a retainer, and two persons each wearing one can engage in a contest. The losers will find themselves unable to resist the urge to obey whatever commands the winners give them (although there are some limits). Somehow SCMs have found their way onto the streets of Japan, and various people are attempting to employ them for one reason or another.

Subaru Mikazuki is a fairly popular author of mystery novels who reads profusely himself. He's eccentric and would rather be reading alone than interacting with other people. He needs an idea for his next work, and thinks that a stray kitten, 'Haru', that he finds one day might provide it.

In the 1930s, Tsugumi Kuze is the heiress of a wealthy family that has fallen on hard times. She agrees to an arranged marriage in order to turn the family's fortunes around. But when her brother meets a horrifying fate, she learns of the 'Imperial Library Intelligence Asset Management Bureau', a secret unit which deals with cursed books that can possess their owners. What's more, the Bureau members discover that Kuze has a unique talent that would be of great use to them, and attempt to recruit her.

One day, Morina Nanase stumbles across a small Italian restaurant, the Trattoria Festa. She is intrigued by the place and is hired to work as a waitress. She finds that the staff have a few quirks, however.